If it is in "like new" used condition I would list it at half it's retail value on Craigslist...
I personally feel 50% of retail should be the normal asking price for :like new" used items... then lower the price from there to account for wear and tear beyond :like new" condition...
I use this guideline whenever buying/selling used aquarium equipment.
Naturally some items will flucuate. For example pumps, filters and what not I am willing to pay a little less than this for, since I can not evaluate the internal wear and tear. Other items that are in higher demand such as 75 gal tanks, I may be willing to pay a little more than 50% retail for...